CS4514 HELP Session 3. Concurrent Server (in APP) Using Go-Back-N (in DLL)

Transcription

1 CS4514 HELP Session 3 Concurrent Server (in APP) Using Go-Back-N (in DLL)

2 Description! You are supposed to implement a simple concurrent server on top of an emulated network protocol stack (NWL, DLL, PHL). Network layer: Packet management only Datalink layer: Go-Back-N sliding window protocol Physical layer: An error module + TCP connection.! Your programs should compile and work on garden.wpi.edu. 12/6/02 WPI Session 3-2

3 Concurrent Server serverbase.txt (1) Server Client1 (3) (2) (4) Child Process1 Client2 Child Process2 Note: each child process keeps a separate copy of the DB. 12/6/02 WPI Session 3-3

4 Framework Client APP Layer NW Layer DLL PHL Server APP Layer NW Layer DLL PHL TCP Connection 12/6/02 WPI Session 3-4

5 Clienti Read scripted action from file scripti.txt APP Server Read/Write a message APP Client Request: cmd No. [msg[ msg] nwl_send ( msg ) nwl_recv ( msg ) Child Process i Note: The max_size of a message is 320 bytes 12/6/02 WPI Session 3-5

6 message Clienti NWL Server NWL message " End of Message dll_send ( pkt ) dll_recv ( pkt ) Note: The max_size of a packet is 60 bytes The network layer will send packets until blocked by the Data Link Layer 12/6/02 WPI Session 3-6

7 Clienti Server packet packet DLL DLL phl_send ( frm ) phl_recv ( frm ) " End of Packet " Error Detection " Byte Stuffing " Go-Back-N Note: The max_size of a frame payload is 32 bytes 12/6/02 WPI Session 3-7

8 Clienti Server frame frame PHL PHL read ( data ) write ( data ) TCP Connection " Identify client when start " Force Single Bit Errors - Client: 6 th frames - Server: 7 th frames 12/6/02 WPI Session 3-8

9 nwl_send ( msg ) nwl_recv ( msg ) Split msg into pkts dll_recv ( pkt ) Pick a pkt Last pkt? No Last pkt? Yes Add sth, \0 No Yes Reassemble pkts into msg Forward msg to APP dll_send( pkt ) 12/6/02 WPI Session 3-9

10 dll_send ( pkt ) Split a packet into payloads Create a new frame Start a Timer Send a frame to PHL Wait for receiving a ACK frame phl_send ( ) phl_recv ( ) Retransmit frames if timeout or error ACK frame! Receive a ACK frame correctly, then continue... 12/6/02 WPI Session 3-10

11 dll_recv ( pkt ) Receive a frame from PHL phl_recv ( ) Unstuffing the frame Compute ED byte and check error Drop if error detected Drop if duplicate, else send ACK phl_send ( ) Reassemble the packet If EOP, forward the packet to NWL 12/6/02 WPI Session 3-11

12 Log Significant Events Performance Timing clienti.log serveri.log Packet Sent Frame received in error 12/6/02 WPI Session 3-12

13 Project Tips! Sliding Window Protocol: Go-Back-N (N>3) Try to implement Go-Back-1 first Then implement Go-Back-N (multiple timers)! Implement non-blocking dll_send() See the example (2 nd next slide) Multi-process programming (maybe) Multi-thread programming (won t suggest)! Maybe easier to merge PHL and DLL 12/6/02 WPI Session 3-13

14 Concurrent Server Example pid_t pid; int listenfd, connfd; listenfd = socket(... ); /* fill in sockaddr_in{ with server's well-known port */ bind (listenfd,...); listen (listenfd, LISTENQ); while(1){ connfd = accept(listenfd,... ); /* probably blocks */ if(( pid = fork()) == 0){ close(listenfd); /* child closes listening socket */ doit(connfd); /* process the request */ close(connfd); /* done with this client */ exit(0); close(connfd); /* parent closes connected socket */ 12/6/02 WPI Session 3-14

15 Relative Timer Example /* example for start_timer, stop_timer, send_packet */ /* you SHOULD modify this to work for project 3, this is just a TIMER EXAMPLE */ #include <stdio.h> #include <stdlib.h> #include <signal.h> #include <sys/time.h> #include <sys/timers.h> #include <sys/select.h> #include <sys/types.h> #include <errno.h> #define TIMER_RELATIVE 0 #define MAX_SEQ 3 extern int errno; typedef unsigned int seq_nr; typedef enum {frame_arrival, cksum_err, timeout, network_layer_ready event_type; timer_t timer_id[max_seq]; 12/6/02 WPI Session 3-15

16 void timeout() { printf( time out!\n ); void start_timer(seq_nr frame_nr) { struct itimerspec time_value; signal(sigalrm, timeout); time_value.it_value.tv_sec = 1; /* timeout value */ time_value.it_value.tv_nsec = 0; time_value.it_interval.tv_sec = 0; /* timer goes off just once */ time_value.it_interval.tv_nsec = 0; timer_create(clock_realtime, NULL, &timer_id[frame_nr]); /* create timer */ timer_settime(timer_id[frame_nr], TIMER_RELATIVE, &time_value, NULL); /* set timer */ void stop_timer(seq_nr ack_expected) { timer_delete(timer_id[ack_expected]); void send_packet(packet *p) { fd_set readfds; int sockfd; 12/6/02 WPI Session 3-16

17 while(packet hasn t been finished sending) { /* send frame if we can */ while(there s place left in sliding window) { /* construct a frame from the packet */ /* send this frame; start timer; update sliding window size */ /* check data from physical layer */ FD_ZERO(&readfds); FD_SET(sockfd, &readfds); if (select(sockfd+1, &readfds, (fd_set *)NULL, (fd_set *)NULL, (struct timeval*)null) < 0) { if (errno == EINTR) { /* receive timeout signal */ /* timeout handler should have resent all the frames that haven t been acknowledged */ continue; else { perror("select error"); /* select error */ exit(1); 12/6/02 WPI Session 3-17

18 if (FD_ISSET(sockfd, &readfds)) { /* a frame come from socket */ /* read a frame from the socket */ if (cksum() == FALSE) { /* error check */ continue; /* do nothing, wait for timer time out */ else { /* check to see if this frame is a data or ACK frame, and do corresponding processing */ continue; 12/6/02 WPI Session 3-18

19 More Issues! The dll_recv() behavior Require received frame buffer! How to terminate client process: When the client gets the response to the quite message A clean way to terminate the server child process? 12/6/02 WPI Session 3-19